In the last few years, use of disposable capsules or cartridges containing an edible powdered substance from which a beverage can be extracted by using hot pressurized water has considerably increased.
In particular, capsules containing a dose of roasted ground coffee for brewing the so-called Italian “espresso” coffee have become quite popular, because the beverage obtained by such capsules is very close in terms of taste and appearance to the espresso obtained by using conventional espresso-type coffee machines normally used in bars and restaurants.
Among those capsules, WO 2005/092160 and WO 2005/092162 disclose a particular capsule which contains a substance in powder form, preferably roasted and ground coffee, the capsule being provided with an elastic septum which generates lasting crema and by means of which it is possible to extract a beverage with improved physical and organoleptic characteristics.
The known elastic septum is fixed downstream of the dose of ground coffee and a filter sheet made of paper or tissue is interposed between the dose and the septum. The septum is basically an elastic membrane which has at least one through orifice or slit provided in a central region and which elastically opens when the pressure inside the capsules exceeds a certain threshold.
A drawback observed in known capsules is linked to the complexity in manufacturing the capsule. In particular, some operations which complicate the manufacturing process are linked to the proper installation of the paper or tissue filter within the capsule, which mainly causes gripping and handling problems because of the filter's flexibility.
Another drawback is due to the fact that capsules which contain ground coffee typically use paper filters in order to retain the solid fractions within the capsule and prevent them from being poured into the external cup. The choice of paper filters entails limitations in the design of the capsule, for example forcing the design of the bottom of the capsule to have no regions in which the filter paper might break under the pressure acting inside the capsule or due to surface unevenness or openings.
A filter in polypropylene was proposed in EP-A 0 309 708. However this filter is characterized by frusto-conical openings and by corresponding nipples protruding towards the substance to be extracted. Therefore, this known rigid filter still suffers from manufacturing problems, because the correct orientation with respect to the powdered substance must be observed when the filter is installed within the capsule.